1) Field of the Invention
The present invention relates to a self-emitting element like an organic electro-luminescent element (hereinafter, “organic EL element”), a display panel, a display apparatus, and a method of manufacturing the self-emitting element.
2) Description of the Related Art
In recent years, self-emitting flat panel displays (hereinafter, “FPD”) in which display panels including light-emitting elements (e.g., organic EL elements) or plasma display panels (hereinafter, “PDP”) are used have been developed. Such display panels include a light-emitting layer disposed between an anode electrode and a cathode electrode. The light-emitting layer emits light when a voltage is applied between the electrodes. The light emitted from the light-emitting layer is recognized as characters or images when viewed through a transparent medium or a transparent panel that has a refractive index more than one.
The light from the light-emitting layer is radiated, or in other words emitted in almost all directions (emitted at all angles). Therefore, light that has an angle of incidence not less than a critical angle with respect to an interface between a transparent medium and an external medium undergo total reflection at the interface and confined in the display panel. For example, it is known that light emitted out of the display panel, which uses the organic EL elements, is only about 20 to 30 percent of total light emitted from the organic EL elements.
One approach to increase the emitted light, i.e., to increase efficiency of the light or increase light extraction efficiency, inclined surface portions may be provided inside the panel. Such inclined surface portions allow light propagating at not smaller than the critical angle to undergo reflection or refraction, and therefore, the light is directed at an angle smaller than the critical angle. Particularly, Japanese Patent Application Laid-open Publication No. 10-189251 discloses a top-emission display panel in which light is emitted from a side of a transparent panel covering a light-emitting layer formed on a base substrate. At a periphery of the light-emitting layer, a wedged reflecting member is disposed, and thereby a reflecting structure is formed. Moreover, Japanese Patent Application Laid-open Publication No. 2001-332388 discloses a bottom-emission display panel in which light is emitted from a side of a base substrate on which a light-emitting layer is formed. In this bottom-emission display panel, a structure in which inclined surface portions are formed on an anode and a cathode which sandwich the light-emitting layer.
However, in such structures in which angle of light emitted from the light-emitting layer is changed by the inclined surface portions and thereby total reflection in the interface is inhibited, the display panel is susceptible to become thick. This may impose restriction on making of a very thin flat panel display. To improve the light extraction efficiency from the display panel, it is necessary to change by the inclined surface portions the angle of light incident on the interface between the transparent panel and the external medium at not less than the critical angle. For this, it is necessary to design a structure such that this light or a major part of this light is incident without fail on the inclined surface. In other words, to reduce light incident on the interface between the light-emitting layer and the transparent panel at the critical angle, it is necessary to form high inclined surface portions. This results in thickening of the transparent panel. Therefore, to improve the light extraction efficiency, a thick transparent panel has to be used. This means that the thickness of the display panel is not reduced. On the other hand, if the transparent panel is made thin, the display panel can be made thin. However, the amount of light incident on the interface at not less than the critical angle increases. This results in deteriorating the light extraction efficiency.
Furthermore, the structure to change the angle of the light does not allow all the light emitted from the light-emitting element to be output from the display panel to the external medium.
In a display panel in which a protective layer is deposited on a display layer that includes the light-emitting element, not all the output light from the light-emitting element is output to the external medium. One of the causes of this is that there is an interface also between the light-emitting element and the protective layer and if the refractive index of the protective layer is lower than the refractive index of the light-emitting element, there is a critical angle when the light is input from the light-emitting element to the protective layer. For example, in a general structure of a display panel in which the organic EL elements are sandwiched between the two electrodes as light-emitting elements, a glass substrate is disposed as a protective layer on the display layer formed on a substrate. The refractive index of glass is 1.5 while the refractive index of the light-emitting particle is 1.7. The light emitted from the light-emitting element and passed through an electrode contains a component that undergoes total reflection at the interface. Therefore, out of the light emitted from the light-emitting element, the light having an angle of incidence on the glass substrate not less than the critical angle is reflected at the boundary of the glass substrate and this light cannot be extracted.
If the refractive index of the transparent panel on the display layer as a protective layer or an output layer is made greater than the refractive index of the light-emitting element, the light that is input from the display layer to the transparent panel does not contain a component of total reflection. Therefore, the light emitted from the light-emitting element can be input to the transparent panel. However, if the refractive index of the transparent panel is increased, the difference between the refractive index of the transparent panel and that of air (i.e., an external medium) becomes greater and the critical angle of the light with respect to an interface between the transparent panel and the external medium becomes smaller. Due to this, even if the transparent panel having a refractive index greater than that of the light-emitting element is used, the amount of light that is extracted in the external medium does not increase. Therefore, the light extraction efficiency cannot be improved.